Just choose which country you want and click the button. Pick whichever location is closest to you, or actually closest to where the content or service you are reaching sits -- for most people that is the same thing, but it is not always, and the account works the same regardless of which one you choose.
Vmess is V2Ray's own protocol. Your client wraps traffic in a WebSocket stream, so to anything watching the connection it has the shape of ordinary web traffic -- and inside TLS on port 443 it is indistinguishable from a normal HTTPS session by its envelope alone.